The field of the invention is radiation therapy systems and methods. More particularly, the invention relates to systems and methods for radiobiological effect modulated external beam radiation therapy.
Radiation therapy is a treatment technique that delivers ionizing radiation to a defined target volume in a patient. Preferably, the radiation is delivered in such a manner that the surrounding healthy tissue does not receive radiation doses in excess of clinically acceptable tolerances. In order to achieve this control of the imparted dose to the subject, highly accurate radiation delivery techniques are required. Many factors provide difficulties in obtaining the desired level of accuracy, including differences between the planned and delivered dose distributions and uncertainty in subject position with respect to the treatment system.
Conventional external beam radiation therapy, also referred to as “teletherapy,” is commonly administered by directing a linear accelerator (“linac”), or cobalt-60 teletherapy unit, to produce beams of ionizing radiation that irradiate the defined target volume in a patient. The radiation beam is a single beam of radiation that is delivered to the target region from several different directions, or beam paths. Together, the determination of how much dose to deliver along each of these beam paths constitutes the so-called radiation therapy “plan.” The purpose of the treatment plan is to accurately identify and localize the target volume in the patient that is to be treated. This technique is well established and is generally quick and reliable.
Intensity modulated radiation therapy (“IMRT”) is an external beam radiation therapy technique that utilizes computer planning software to produce a three-dimensional radiation dose map, specific to a target tumor's shape, location, and motion characteristics. Various regions within a tumor and within the patient's overall anatomy may receive varying radiation dose intensities through IMRT, which treats a patient with multiple rays of radiation, each of which may be independently controlled in intensity and energy. Each of these rays or beams is composed of a number of sub-beams or beamlets, which may vary in their individual intensity, thereby providing the overall intensity modulation. Because of the high level of precision required for IMRT methods, detailed data must be gathered about tumor locations and their motion characteristics. In doing so, the radiation dose imparted to healthy tissue can be reduced while the dose imparted to the affected region, such as a tumor, can be increased. In order to achieve this, accurate geometric precision is required during the treatment planning stage. Thus, while conventional IMRT methods have had success in increasing the physical dose imparted to the defined target volume while mitigating the imparted radiation dose to the surrounding healthy tissue, further reduction of the radiobiological effect on healthy tissue is desirable. Particularly, while IMRT has effectively reduced the physical absorbed dose of radiation to sensitive areas within a patient, there is still room for improvement in reducing the biological effect of such radiation.
In general, methods of producing intensity modulated rays of radiation are well known in the art. Exemplary methods include (1) stop and shoot methods, such as the one described by P. Xia and L. J. Verhey in “Multileaf Collimation Leaf Sequencing Algorithm for Intensity Modulated Beams with Multiple Static Segments,” Medical Physics, 1998; 25:1424-1434; (2) sliding window methods, such as the one described by T. Bortfeld, et al., in “Realization and Verification of Three-Dimensional Conformal Radiotherapy With Modulated Fields,” Int'l J. Radiat. OncoL Biol. Phys., 1994; 30:899-908; (3) intensity modulated arc therapy methods, such as the one described by C. X. Yu in “Intensity-Modulated Arc Therapy With Dynamic Multileaf Collimation: An Alternative to Tomotherapy,” Physics in Medicine & Biology, 1995; 40:1435-1449; and (4) sequential (axial) tomotherapy methods, such as the one described by M. Carol, et al., in “The Field-Matching Problem as it Applies to the Peacock Three Dimensional Conformal System for Intensity Modulation,” Int'l J. Radiat. Oncol. Biol. Phys., 1996; 34:183-187.
Image-guided radiation therapy (“IGRT”) employs medical imaging, such as computed tomography (“CT”), concurrently with the delivery of radiation to a subject undergoing treatment. In general, IGRT is employed to accurately direct radiation therapy using positional information from the medical images to supplement a prescribed radiation delivery plan. The advantage of using IGRT is twofold. First, it provides a means for improved accuracy of the radiation field placement. Second, it provides a method for reducing the dose imparted to healthy tissue during treatment. Moreover, the improved accuracy in the delivery of the radiation field allows for dose escalation in the tumor, while mitigating dose levels in the surrounding healthy tissue. The concern remains, however, that some high-dose treatments may be limited by the radiation tolerance of healthy tissues that lay close to the target tumor volume.
It would therefore be desirable to provide a method for performing external beam radiation therapy that permits high levels of dose to a target volume of interest while further controlling damage to healthy tissue and organs at risk surrounding the target volume being treated.